The update mode. In ‘force’ mode, self.datas is cleared and all
updates are redone. In ‘update’ mode, existing data are preserved
and new can be added. The ‘normal’ mode depends on other
attributes: for stationary (self.kind=='stationary')
materials and materials in ‘user’ mode, nothing is done if
self.datas is not empty. For time-dependent materials
(self.kind=='time-dependent', the default) that are not
constant, i.e., are given by a user function, ‘normal’ mode behaves
like ‘force’ mode. For constant materials it behaves like ‘update’
mode - existing data are reused.